1. Field of the Invention
The present invention relates to a substantially reactive and gel-free composition for making hybrid composites.
2. Prior Art Related to the Invention
In the 1990s, sol-gel chemistry has been used extensively to produce organic-inorganic composites. A number of patents and published articles have reported a variety of synthetic routes by using this chemistry and technology for preparation of hybrid composites. In general, these sol-gel derived hybrid composites can be divided into two basic classes: those where particles are formed in situ or those where particles are directly employed as starting materials.
In the first class of hybrid composites, organosilanes and/or other metal alkoxides are employed not only as particle-precursors but also as network-formers. Very often, the mixtures of several types of precursors are used. In the presence of water, solvent (i.e. alcohol), and also catalysts (acid or base), simultaneous hydrolysis and condensation of these organosilanes and/or other metal alkoxides take place to form inorganic sols mixed with inorganic/organic networks, therefore, hybrid composites.
Usually, in order to obtain better processability, hybrids with highly organic characteristics are desired. To achieve this, more organic components, such as monomers, oligomers or polymers, are incorporated into the precursor solution first, and then hydrolysis, condensation and polymerization/cross-linking reactions are carried out.
As a representative example, U.S. Pat. No. 6,001,163 demonstrated the compositions and method to make this class of composite. An epoxy functional silane is used to provide polymerizable functional groups, and thus is an organic network former; TEOS (tetraethoxysilane) is used as a precursor for both particles and inorganic networks. Multifunctional carboxylic acids (anhydrides) or their combination are used as catalysts. The hybrid materials produced show good abrasion resistance. In U.S. Pat. No. 5,316,855, the organic/inorganic hybrid composites are prepared by co-condensing metal alkoxide sols (e.g. aluminum, titanium, or zirconium alkoxide sols) with one or more bis (trialkoxysilane-containing) organic components. The new hybrid composites show optical clarity and improved abrasion resistance. A number of patents, such as U.S. Pat. No. 6,071,990, 5,120,811, 5,548,051, WO 00/29496, EP 1,016,625, etc., are all believed to belong to this class.
In general, neither hydrolysis nor condensation reactions can be completed unless a high temperature process is applied. As a result, unreacted hydroxyl and alkoxyl groups remain in the produced materials, as illustrated in U.S. Pat. No. 5,316,855. Therefore, both hydrolysis and condensation of these reactive groups are expected to continue for a long time until a dynamic equilibrium is reached.
In the second class, such as in U.S. Pat. No. 4,455,205, 4,478,876, 4,491,508, 6,160,067, and EP 0,736,488, etc., pyrogenic or precipitated particles (e.g. SiO2, Al2O3) are used as starting materials. The particles are first dispersed into organic media, usually hydrophilic solvents, such as alcohols. Then organo-functional silane(s) with necessary water and catalysts are added. The grafting reactions take place on the surface of the particles. Finally, the surface modified particles are mixed into the polymeric matrix or reactive monomers/oligomers to form organic-inorganic hybrid composites after polymerization/cross-linking.
A typical example demonstrated in U.S. Pat. No. 4,624,971 (Battelle), an abrasion resistant UV curable composition for coating substrates was produced. In the first step, pyrogenic silica or alumina particles having a particle size of less than 100 nm are dispersed into organic solvents. Then, by mixing hydrolyzed trialkoxysilanes with the particle dispersions, methacryloxypropyl, or glycidoxypropyl, or epoxycyclohexyl reactive groups are chemically bonded on the surface of the particles. Here, hydrolyzed trialkoxysilanes serve as both surface-modifying agents and inorganic network formers. The amount of these silanes is usually greater than 20% in the total composition weight.
In general, often only one of three, sometimes two of three silanol groups of hydrolyzed trialkoxysilanes is/are bonded on the surface of the particles. This bonding limitation is the result of both the limited reactivity and the steric effect of the silanols. In this regard, see Brinker et al., “Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing”, pp. 236—269, 1990 (Academic Press, Inc.). Again, unhydrolyzed alkoxyl and uncondensed free hydroxyl groups can cause the same problems described previously. Moreover, agglomeration of functionalized particles can also take place through the formation of Si—O—Si and/or hydrogen bonds located on the surface of particles.
For a low concentration, solvent-based application, this should not be a big issue because the solvent dilution keeps particles separated, and thus limits the formation of either large particles or networks.
In order to satisfy increasingly rigorous environmental regulations and meet high performance requirements, it is often desirable to use substantially reactive materials, such as radiation curable materials, or at least, low solvent-containing materials. Therefore, it is necessary to produce a stripped material by removing both water and solvent (alcohol) azeotropically. In contrast to the case of solvent-dilution, the remaining alkoxyl and silanol groups have a much higher probability of contacting each other during the concentration operation. As a result, these slow hydrolysis and condensation reactions cause a gradual extension of inorganic networks through siloxane, Si—O—Si, bonds and/or hydrogen bonds. Consequently, unstable viscosity, large particle formation and even gel formation occur. This is a significantly troubling issue for all practical large-scale productions.
It is well known that in the case of free radical radiation curable acrylates and methacrylates, removing oxygen inhibition during the solvent-stripping operation can also cause the gelation. However, this gelation is essentially different from one caused by the silanol condensation reactions. The former one is the result of free radical polymerization of acrylates or methacrylates. Either air sparging or the addition of extra free radical inhibitor can prevent this gelation.
U.S. Pat. No. 5,103,032 relates to compositions containing an acrylsilane or methacryloxysilane and an N,N-diakylaminomethylene phenol in an amount at least sufficient to inhibit polymerization of the silane during its formation, purification and storage.
U.S. Pat. No. 5,817,715 relates to a gel-free silica acrylate UV curable coating composition. This coating material is composed of one or more of soluble salts, soaps, amines, nonionic and anionic surfactants, etc., and a similar sol-gel composition described in U.S. Pat. No. 4,624,971. No radiation curable salts are mentioned. In addition, the water-soluble additives that are mentioned may cause more hydrolytic stability problems.
In view of the foregoing, an objective of the invention is to provide compositions for hybrid composite materials, which compositions are solvent free or with a very low-level of solvent, unlike traditional compositions for hybrid composite materials, which are prepared via a sol-gel process.
Another objective of the invention is to provide compositions for hybrid composite materials with better Theological behavior, therefore, better processability than that of traditional compositions for hybrid composite materials prepared via sol-gel process.
Another objective of the invention is to provide compositions for hybrid composite materials, which compositions have stable viscosity, therefore, better processability, than that of traditional compositions for hybrid composite materials, which compositions are prepared via a sol-gel process.
Another objective of the invention is to provide compositions for hybrid composite materials that are radiation (UV/electron beam) curable.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with better surface hardness than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with better surface scratch resistance than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with better abrasion resistance than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with better solvent/chemical resistance than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with higher impact resistance than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with higher storage modulus than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with higher loss modulus that those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials that form cured coatings/films with higher Tg (glass transition temperature) than those formed solely from base-resins.
Another objective of the invention is to provide hybrid composite materials than form cured coatings/films with better weatherability than those formed solely from base-resins.